May 6, 2020
Pulse Code Modulation, or PCM, is the core technology behind digital audio. Despite being so central to modern day life PCM is actually pretty old. It was originally developed by Alec Reeves, a telecom engineer, all the way back in 1937. From there the technology has slowly gained traction and eventually found its way into computerized audio systems. Decades after its creation Reeves would write out his thoughts on the technology in an article titled "The past, present and future of PCM"(https://tkhf.adaxas.net/cd1/Reeves2.pdf). Despite being written in 1965, more than 50 ago, his predictions for the future are uncannily accurate.
Reeves uses the year 2000 as a goalpost for most of his long-term predictions. Some of these are pretty mundane: telephone usage will greatly increase, new technology will spread in emerging markets outside the US and Europe. His safest prediction is that larger and more widespread use of telephone systems will make PCM the only suitable option for ferrying audio. But Reeves would go on to describe other causes for widespread adoption of PCM:
"In my view, this “other reason” by that date will be the necessity for widespread closed-loop television — a necessity, I repeat, not just the urge for a status symbol that is likely to start this kind of demand in the nearer future."
I've struggled a bit to understand exactly what he means by "closed-loop television" here. It's clear he isn't referring to CCTVs, the direct context makes me think he means broadcast television. However, later in the paper when addressing the use of PCM in information retrieval he writes:
"The only adequate answer will be for a few information processing centers to be set up in each large industrialized area, staffed by top-grade people, with the information being made available to the public immediately and automatically when a dialed request is made. An ordinary high-speed data link may be adequate for the next 20 years, but by A.D. 2000 the only way to pass the information fast enough to the caller’s brain will be to use moving pictures."
So it could be that he is trying to describe a teleconferencing-like system. Whatever the case, Reeves is still accurate in predicting the audio side of things. Broadcast television signals switched to digital in most parts of the world during the 2000's, and the audio component of those signals is now encoded as PCM. However, I think the teleconferencing angle has a little more meat to it. In a later passage Reeves writes:
"Commuters will refuse to accept the delays and inconveniences that even a moderate journey to and from their place of work would entail...We shall have to transport the brains, the skills of the staff, not their bodies, to their daily jobs, again involving not merely ordinary data links but a great many private television channels as well."
The language is a little anachronistic, but here Reeves is speculating that video conferencing will become essential as more employees wish to work remotely. Of course today we don't conference using a TV, we use computers. But just like digital television signals, video conferencing software like Skype employs PCM for audio encoding.
To learn more about the story of PCM, listen to my episode on the topic: